Recently, as electronic devices are highly densified, circuit substrates such as a flexible print wiring board used in those electronic devices are multi-layered. As one of the laminating techniques for forming a multi-layer circuit substrate, a build-up method has been employed. The build-up method is a method for laminating a resin layer formed of only a resin and conductive layers so that the interlayer connection is established between the conductive layers through the single resin layer.
In general, this build-up method includes a method for interlayer-connecting conductive layers after via holes are formed on a resin layer and a method for laminating a resin layer after joint portions are formed on a conductive layer. The joint portions between the conductive layers are formed either by plating the via holes or by using a conductive paste.
As a technique capable of forming a stacked via structure and achieving densification and simplification of the wiring design, there is known a method which accomplishes electrical connection between conductive layers by forming micro via holes for the electrical connection on a resin layer by laser irradiation and filling the via holes with a conductive adhesive such as copper paste, in which the conductive adhesive provides the electrical connection between the conductive layers (see, for example, Japanese Patent Laid-Open Publication No. Hei 8-316598).
However, in this method, since the electrical connection between the layers is accomplished by the conductive adhesive, the reliability is often not sufficient. Also, in order to fill the micro-via holes with the conductive adhesive, a highly advanced technique is required. Further, since the conductive adhesive has a relatively high on-resistance, when it is filled in the micro-via holes, the on-resistance is increased. Therefore, it is difficult to use such a conductive adhesive for the electrical interlayer connection between substrates where a wiring is made at a high density.
Accordingly, instead of the technique of filling the via-holes with the conductive adhesive, a technique of using metallic protrusions (conductive posts) has been used. As the technique of using the conductive posts, there is known a method of mechanically connecting the conductive posts with a conductive pad without using an interlayer adhesive (see, for example, Japan Patent Laid-Open Publication No. Hei 11-54934).
However, this method involves a problem in that it is difficult to completely eliminate the interlayer adhesive between the conductive posts and the conductive pad, such that the reliability is not sufficient.
Also, when the structure is re-heated for the mounting of parts, after the interlayer connection is formed once, there is a case that the solder formed for the interlayer connection is re-melted, deformed and moved, thereby deteriorating the reliability of the interlayer connection.